1. Field of the Invention
This invention relates to an apparatus for controlling hydraulic pressure which operates a clutch used in changing speed of a working vehicle and to a clutch pressure control method.
2. Description of the Related Art
A conventional working vehicle is shown in Patent Laying-Open Publication H6-11023, for example. In this conventional vehicle, one of a plurality of predetermined pressure curves is selected based on such factors as rotating speed of the engine, a change speed shift position and rolling of the wheels. When a hydraulic clutch (change clutch) is engaged, the pressure of the clutch is controlled to increase according to the pressure curve selected.
Where, as in the conventional working vehicle, the pressure is increased according to the characteristic selected when the hydraulic clutch is engaged, one of predetermined pressure increase characteristics is selected. However, this requires numerous pressure characteristics must be set in order to engage the hydraulic clutch smoothly while coping with various factors. Such a working vehicle is troublesome to manufacture, and has room for improvement.
Besides, where the pressure of the hydraulic clutch is controlled according to one of the pressure curves stored, the clutch pressure could be increased gradually according to the pressure curve, for example, even after the working vehicle reaches a desired speed. In such an event, an unduly long time is taken before completion of clutch connection, i.e. clutch change speed. That is, the clutch pressure is controlled regardless of a state of the working vehicle. Such clutch pressure control cannot be said optimal. There is room for improvement in this respect also.
A shock of change speed occurs when the hydraulic clutch is engaged. Due to mechanical variations of the hydraulic clutch or mechanical variations of an electromagnetic valve or the like for controlling the pressure of the hydraulic clutch, even a proper control may fail to increase the pressure of the hydraulic clutch according to a selected one of the predetermined characteristics. In this sense also, there is room for improvement.